I feel like Cthulhu Wars moves very quickly as long as everyone is engaged. As the player count climbs, it does become harder to make sure everyone is engaged, so the time per turn seems to climb. I think it's pretty great as is.Play by forum can drag if it gets too drawn out. People then slow to respond even moreso.
My vote for a quick game would be four players without HPs, iGOOs, or Neutrals. Also without a heavy doom phase map.